1. Field of the Invention
The present invention is applied to a base station of a cellular system which uses the CDMA (Code Division Multiple Access) as a communications access method and relates to a receiver which uses the DBF (Digital Beam Forming) for digitally executing signal processing as an antenna technology and a demodulator applied to such a receiver.
2. Discussion of the Background
In recent years, a method called the CDMA (Code Division Multiple Access) has been developed as a communications access method in the next-generation cellular system. In the CDMA system, a single frequency band is accessed by a plurality of users and distinction between users is made via user-specific codes.
According to a cellular system of the CDMA type, the same frequency band is shard by a plurality of users, thus improving the frequency use efficiency compared with the TDMA (Time Division Multiple Access) and FDMA (Frequency Division Multiple Access).
Recently, a further increase in subscriber accommodation is requested, considering a rapid increase in the number of subscribers in future. In order to satisfy this request, application of the CDMA is under examination. In this case, to make most of the CDMA features, it is required to suppress cochannel interference caused by the fact that the spreading codes of stations in the same cell are not orthogonal in a strict sense, and interference from a mobile station in another cell using the same frequency.
Thus, use of the Digital Beam Forming, hereinafter referred to as the DBF, for a cellular system of the CDMA type is under examination. The DBF performs weighed synthesis of signals received via a plurality of element antennas to orient the antenna beam (directivity) to the direction of desired waves or direct the null point of antenna to the direction of incoming unwanted waves.
The DBF can suppress the receiving power of the unwanted waves to a very small level, thus suppressing cochannel interference and adjacent channel interference. Thus, application of the DBF to a cellular system of the CDMA type can reduce interference amount within the same cell. This can relieve reduction of subscriber capacity caused by interference from another user.
A conventional example of configuration in which the DF is applied to a cellular system of the CDMA type is disclosed in xe2x80x9cCHARACTERISTICS OF DIVERSITY ADAPTED FOR JUDGEMENT FEEDBACK TYPE COHERENT IN DS-CDMA, Tanaka et al., shinngaku giho, RCS-96-102 (1966-11).xe2x80x9d
FIG. 7 is a block diagram of the internal configuration of a receiver disclosed in the aforementioned conventional document. This receiver is of a so-called element space type which executes weight control processing for controlling directivity. The receiver comprises a combiner 90 for performing synthesis of the output of a plurality of fingers 80 provided for a plurality of paths and each finger 80. Output of the combiner 90 is used for data decision.
The finger 80 has a matched filter (MF) 81. The matched filter 81 performs despreading of a digital output signal respectively corresponding to each element antenna to obtain a demodulated signal. The demodulated signal is given to a beam forming section 82. The beam forming section 82 has a plurality of weight operation sections 83.
Each weight operation section 83 multiplies a demodulated signal corresponding to each element antenna by a weight coefficient given by a weight controller 84. Output of each weight operation section 83 is synthesized by the synthesizer 85. As a result, a composite demodulated signal is generated. The composite demodulated signal is equivalent to a signal received via an antenna having a directivity showing a high receiving sensitivity to desired waves and a low receiving sensitivity to interference waves.
The composite demodulated signal is given to a phase compensator 86 as well as to a phase estimator 87. The phase estimator 87 determines a phase estimate based on the composite demodulated signal. The phase compensator 86 executes phase compensation control to compensate for fading on a propagation path on the composite demodulated signal, based on the phase estimate determined via the phase estimator 87. As a result, the phase of the composite demodulated signal is adjusted. The composite demodulated signal which has undergone the phase compensation are given to the combiner 90.
In the aforementioned technology, however, weight processing and phase compensation processing on the demodulated signal corresponding to each element antenna are made by separate circuits, that is, by the weight operation section 83 and the phase compensator 86 respectively. This resulted in a larger circuit scale.
An object of the invention is to provide a receiver which allows reduction of circuit scale, applicable to a mobile communications system and a demodulator provided in the receiver.
The invention to attain the object is a receiver applied to a mobile communications system, the receiver comprising an antenna, and A/D converter for converting antenna output signals from the antenna to digital signals, a path finder for finding a propagation path, a plurality of fingers which input digital signals output from the A/D converter, and a synthesizer for synthesizing output signals from the plurality of fingers, characterized in that the fingers comprise restoration means for performing despreading of a digital signal corresponding to the propagation path found by the path finder among digital signals output from the A/D converter to generate a demodulated signal, phase estimation means for determining a phase estimate based on a demodulated signal generated by the restoration means, and compensation means for multiplying the demodulated signal by a weight/phase compensation coefficient for simultaneously performing weight control and phase compensation control obtained based on the phase estimate determined via the phase estimation means to obtain an output signal for the finger.
The antenna preferably comprises a plurality of element antennas and preferably further comprise a restrained beam former for forming a plurality of sub-beams oriented to a plurality of directions and outputting a signal corresponding to each sub-beam as an antenna output signal. In this case, the A/D converter preferably converts an antenna output signal corresponding to each sub-beam output from the restrained beam former to a digital signal.
The antenna preferably comprises a plurality of element antennas and the A/D converter preferably converts an antenna output signal to a digital signal. In this case, restoration means, phase estimation means and compensation means included in the fingers are provided for each element antenna and correspond to each element antenna and preferably process a digital signal corresponding to a propagation path found via the path finder.
The invention is a demodulator provided in a receiver applied to a mobile communications system, the receiver comprising a feature to generate digital signals corresponding to antenna output signals, a feature to find propagation paths based on the antenna output signal, and a feature to synthesize finger output signals, the demodulator having a plurality of fingers, characterized in that the fingers comprise restoration means for performing despreading of a digital signal corresponding to the propagation path found by the path finder among digital signals output from the A/D converter to generate a demodulated signal, phase estimation means for determining a phase estimate based on a demodulated signal generated by the restoration means, and compensation means for multiplying the demodulated signal by a weight/phase compensation coefficient for simultaneously performing weight control and phase compensation control obtained based on the phase estimate determined via the phase estimation means to obtain an output signal for the finger.
According to the invention, weight control and phase compensation control are simultaneously performed via a single compensation means, based on a weight/phase compensation coefficient. Thus, circuit scale can be reduced compared with the case in which weight control and phase compensation control are made via separate circuits. This allows a lower-cost receiver to be provided.
Moreover, according to the invention, a receiver or a demodulator preferably further comprises a single compensation coefficient operator section for determining the weight/phase compensation coefficient in a time division fashion for each finger based on the phase estimate determined via the plurality of fingers, and giving each determined weight/phase compensation coefficient to each finger.
According to the invention, a plurality of weight/phase compensation coefficients used by fingers are determined by a single compensation coefficient operation section. Thus, circuit scale can be reduced compared with the case in which a compensation coefficient operation section is provided for each finger. This allows a further lower-cost receiver to be provided.